Amidst the recent competition of manufacturers to reduce the thickness of the interior and exterior materials of electronic products for home appliances such as PDPs, LCDs, LED TVs, etc., injected plastic products which have been conventionally used for the interior and exterior materials are being rapidly replaced by thin iron steel plates. Particularly in the case of exterior materials, a resin coated steel plate such as a black resin steel plate the surface of which is beautiful and is colored is being applied so that it is able to be mounted directly onto products after pressing without additional painting.
Typically, a resin coated steel plate has a structure of top coat/primer/zinc plated steel plate/primer, and the top coat layer is composed mainly of a polyester resin and a melamine resin with a thickness 10˜50 μm, and the primer layer is composed mainly of an acrylic urethane ester resin and has a thickness of 0.5˜1.0 μm.
Such a resin coated steel plate may be applied to exterior materials directly after pressing without having requiring additional painting, and it is produced to have a blackness of about 75˜98 because the color of the top coat layer prefers black (blackness=100−L*, L*=whiteness) in terms of surface appearance.
The color of the resin film is determined by the properties of a pigment composed mainly of carbon black and the film thickness properties. As the amount of the black pigment increases and the thickness of the film increases, black color is observed with the naked eye due to the absorption of light, that is, the blackness is increased. The case where the blackness (100−L*) is 80 or more is regarded as “real black”, the case where the blackness is 80˜50 is regarded as “black gray”, and the case where the blackness is 50 or less is regarded as “gray”. The ability of the resin coated steel plate to absorb/radiate heat may increase depending on the blackness based on the composition properties of a paint resin for use in the pigment that exhibits the blackness. The heat transfer is possible by direct heat transfer through an object, heat convection, and radiant heat, and absorption/radiation by radiant heat may increase in proportion to the increase in blackness. In particular, in back-cover material fields for image displays, black (real black) is preferred because of “beautiful factors” and “heat emission from an image device”.
In order to enhance the bondability between the top coat layer and zinc metal, the primer layer comprises an organic/inorganic composite film composed mainly of acrylic urethane ester. To increase productivity in a continuous process for producing the resin coated steel plate thus structured, it is favorable that the thickness of the top coat layer be decreased. However, as the thickness of the top coat layer (the resin layer) is decreased, the zinc plated layer may be transferred to the surface of black resin, undesirably reducing blackness and corrosion resistance.
Culminating in the present invention, intensive and thorough research was carried out by the present inventors aiming to solve the problems of blackness and drawing ability being reduced in proportion to a decrease in the thickness of the resin layer as encountered in the related art. The research led to the development of a black resin steel plate having improved blackness and drawing ability by decreasing the coefficient of friction of a black resin film.
Accordingly, an object of the present invention is to provide a black resin steel plate in which the transfer of a resin layer due to a reduction in thickness of the resin layer may be blocked and blackness and drawing ability may be improved, and also to provide a method of manufacturing the same.